Here we study the least-bound vibrational level of the X\(^1\Sigma _g^+\)  electronic ground state of the \(^{86}\) Sr\(_2\)  dimer (Fig. REF ), which is a naturally occurring halo molecule, meaning it exists in the absence of tuning with a magnetic Feshbach resonance. A well-known example of a naturally occurring halo molecule is the \(^4\) He\(_2\)  dimer [1]}, [2]}, [3]}. The least-bound vibrational level of the ground state of \(^{40}\) Ca\(_2\) , which was recently studied using similar methods [4]}, is also very close to this regime.
